In direct conversion and zero-IF (Intermediate Frequency) receivers, it is necessary to maintain the spectral purity of the channel used for reception. Because of limited narrow band selectivity, second order intermodulation distortion (IM2) presents an undesired spectral component within the signal band of interest. This occurs when two or more interfering signals, whose difference in frequency is less than the IF bandwidth of the desired signal, mix with one another due to some second order nonlinearity and produce a baseband spectral component. To minimize the effects of second order intermodulation within critical circuit blocks in the signal path, it is known in the art to use differential circuits. In theory, differential circuits have infinite attenuation to second order intermodulation distortion. In reality, this is far from the truth; due, in no small part, to device mismatches, parametric imbalance, imperfect layout, and other device characteristic inequalities that cause imbalances and provides a lower than desired second order input intercept point (IIP2).
IM2 and IIP2 are further defined in EP 1 111 772, for example.
EP 1 111 772 discloses several embodiments of a mixer circuit. Most of these embodiments are double-balanced mixer circuits that need a differential RF (Radio Frequency) signal as inputs.
In FIG. 8 of EP 1 111 772 a single balanced mixer circuit is disclosed that needs not a differential RF signal as inputs. This single balanced mixer circuit mixes the RF signal with a LO (Local Oscillator) signal to generate an IF (Intermediate Frequency) signal. This single balanced mixer circuit has a mixing branch including:
a transconductance stage having an RF input to receive the RF signal to transform into a current signal, and
a current switching core to switch the current signal according to the LO signal and an LO signal which forms with the LO signal a differential signal.
However, this single-balanced mixer circuit does not properly reject the LO signal or harmonic of the LO signal in the IF signal.